Field of the Invention
The present invention relates to a circuit board, an electronic apparatus, and an image forming apparatus.
Description of the Related Art
In recent years, image forming apparatus have been reduced in size and increased in functionality. As a result, electrical circuits are naturally mounted at high density, and in many cases, a circuit board is divided to be arranged in remaining space in a product in order to reduce the overall size of the product. The remaining space is, for example, a minute gap immediately above one board, a gap between mechanical mechanisms, a space in the middle of a path of bundled cables, or a space just inside the exterior, that is, a location at which the product is adjacent to another apparatus when that apparatus is installed adjacent to the product. In such a case, the circuit boards are close to each other, and hence there is a fear in that noise generated at a certain circuit or member enters peripheral circuits. In particular, transformers that emit strong electromagnetic energy and high-frequency digital signals affect the peripheral circuits. Hitherto, measures against noise entrance need to be provided only to the periphery of components on the same board and adjacent patterns. However, even when boards are separated from each other, it is necessary to consider that noise generated at a board may enter another board arranged physically very close to that board. As a technology for shielding noise not in a horizontal direction but in a vertical direction with respect to a board surface as described above, a shield using a metal plate is generally used. For example, there is a method in which the frame of a product is deformed to function as a shield. However, the frame is thick to maintain its strength, and it is expensive to use the frame only for a shield for noise in terms of material and processing costs. A shield for noise only needs to be capable of shielding electric fields. Thus, in a case of a multilayer board, for example, a sufficient shielding effect is obtained when a ground pattern in which the entire region of a surface layer is subjected to conductive coating is employed. However, a shield cannot be formed on a single-sided board including a paper phenolic board (hereinafter referred to as “FR-1 board”), which is inexpensive and thus widely used.
As a shield that may be applied to such a board, for example, there is a configuration in which a flexible printed circuit connected to a ground, which is called “sub-printed wiring board”, is formed on a board that is a noise source (for example, see Japanese Patent No. 3563522). With such a configuration, the board is entirely shielded. In particular, cuts or slits are formed at locations corresponding to components mounted on the surface of the board that is the noise source, and thus the sub-printed wiring board can be bonded to no-component mounted portions of the board that is the noise source without a gap. Noise can therefore be shielded.
However, in the related art, the flexible printed circuit is used as the sub-printed wiring board for shielding, which is a problem. In the rerated art, the sub-printed wiring board is made of polyimide, for example. Polyimide is generally expensive. When polyimide is used for a shield for an FR-1 board, the shield is more expensive than the FR-1 board itself. Further, the holes or the slits are formed in the sub-printed wiring board at locations corresponding to the components on the board that is the noise source, and consequently, when the arrangement of the components on the board that is the noise source is changed, the design of the sub-printed wiring board also needs to be changed. Further, the sub-printed wiring board is soldered to the ground of the board that is the noise source, and is bonded to the board that is the noise source using an adhesive. Thus, there is a problem of a large number of assembly steps.